Conventionally, as shown in FIGS. 40(a) and 40(b), a linear heater 23 like a feeder line of TV, etc. has been used to prevent freezing of water in a water pipe at the coldest of the year in cold districts. The linear heater 23 is composed of a linear main body 22 in which a metallic resistance wire 21 such as a nichrome wire is coated with a vinyl chloride resin or the like which is an electrical insulator.
The linear heater 23 is used such that, for example, the main body 22 is wound around an outer surface of a water pipe in a spiral or is contact-bonded along the water pipe in its lengthwise direction so that the linear heater 23 closely adheres to the outer surface of the water pipe which is exposed on the ground in a house in clod districts. In order to prevent freezing, the linear heater 23 is set so that a fixed amount of electricity, for example, 6 W of electricity per 1 m of the water pipe is consumed.
In addition, a sensor, which controls on/off operations of the linear heater 23 according to a detection temperature, for example, a platinum sensor is provided to a position which is close to an exposed portion of the water pipe in the coldest place of a house, i.e., usually, the north side of the house.
When the detection temperature detected by the sensor becomes lower than 0.degree. C., the linear heater 23 generates heat by electrifying the metallic resistance wire 21 so as to heat the outer surface of the water pipe around which the linear heater 23 was wound. As a result, the water pipe is prevented from bursting due to freezing of water at a low temperature, for example, the detection temperature is lower than 0.degree. C.
However, the linear heater 23 shall be controlled so as to be turned on/off based upon the detection temperature by the sensor provided to a higher position such as a north side of a house where a temperature becomes the lowest.
For this reason, in the case where the linear heater 23 is installed to a water pipe along a south side of a house, for example, even when an outside air temperature on the south side of the house rises and the water temperature in the water pipe rises to such a temperature that freezing of the water does not have to be considered, the linear heater 23 is in the ON state as long as the detection temperature by the sensor is lower than 0.degree. C. For this reason, the linear heater 23 is occasionally in the ON state all day long for a portion which does not have to be heated.
As a result, since the linear heater 23 cannot collectively heat only a specified portion where the temperature of a water pipe is low, it occasionally consumes useless electricity. Thus, the linear heater 23 has such a problem that the consumption of electricity is increased.
Therefore, in order to solve the above problem, it was considered that a heating cable disclosed in U.S. Pat. No. 4,072,848 is used as a heater for preventing freezing of water in a water pipe, etc. at the coldest of the year in cold districts.
In the above publication, as shown in FIG. 41, heating elements 52 with a chip configuration which generate heat by electrifying, and a pair of electric wires 53 made of copper which feed electricity to the heating elements 52 are sealed into a cable-like main body 51 composed of an insulator such as a thermoplastic resin.
The heating elements 52 are positive-characteristic thermistors composed of barium titanate ceramics, and they respectively have the electrode 54 for ohmic contact on both the sides of the main body 51 in the lengthwise direction.
A plurality of the heating elements 52 are placed between the electric wires 53 at fixed intervals along the lengthwise direction, and the heating elements 52 respectively have the electrodes 54 and joints 55. The joints 55 come into contact with the side surfaces of the electrodes 54 and electrically connect the electrodes 54 to the electric wires 53 by soldering along the sides of the electric wires 53.
Such a heater is used with the main body 51 coming into contact with a water-residence portion of a water pipe, etc., the heating elements 52 generate heat according to a temperature so as to be able to prevent a damage of the water-residence portion due to freezing of water at a low temperature, for example, when the temperature in the water-residence portion is lower than 0.degree. C.
In other words, when the temperature of the heating elements 52 become, for example, not more than 5.degree. C., they are electrified so that heat is generated. As a result, the water-residence portion, which comes into contact with the main body 51 close to the heating elements 52 which generated heat in such a manner, is heated through the main body 51 by the heating elements 52, so a damage of the water-residence portion due to freezing of water can be prevented in the surrounding of a low temperature where the water temperature in the water-residence portion is lower than 0.degree. C.
However, the heater disclosed in the above publication is usually wound around the outer surface of a water-residence portion of a water pipe, etc. having a big curvature. In such a case where the heater is bent, a strong bending stress is applied to the main body 51, and the bending stress is applied also to the joints 55 which electrically connect the heating elements 52 to the electric wires 53.
At this time, since the electric wires 53, which are sealed into the main body 51 and made of copper, have ductility, they can be bent along the deflection of the main body 51 made of a thermoplastic resin so that the influence of the bending stress can be avoided. However, the heating elements 52 and the joints 55 are hard because they are formed by ceramics and solder. For this reason, since heating elements 52 and the joints 55 cannot be bent according to the bending stress, the bending stress is applied to them.
Accordingly, since the above-mentioned conventional heater is used for preventing water in the water-residence portion from freezing, a temperature changes greatly. Therefore, while the temperature changes frequently and thus thermal expansion and thermal contraction are repeated on the joints 55, a crack might occurs between the joints 55 and the electrodes 54, which are connected to the joints 55, or the heating elements 52 due to the strong bending stress.
As a result, the electrical connection between the heating elements 52 and the electric wires 53 via the joints 55 cannot be maintained, and the electric resistance value between the heating elements 52 and the electric wires 53 rises. Therefore, heating to a subject to be heated from the heating elements 52 becomes unstable.
The present invention is invented in view of the above problems, and it is an object of the present invention to provide a heater, which is capable of preventing water in a water-residence portion of a water pipe, etc. from freezing by suitable heating and capable of making heating in use stable, and to provide a manufacturing method of the heater.